1. Field of the Invention
The present invention relates to a screw and a screwdriver, and more particularly to use for attachment of parts of, for example, home electric appliances, electric device parts, office machines such as copying machines, and optical apparatus or precision apparatus such as digital camera.
2. Description of the Related Art
Recesses (holes) are disposed on a top surface of a screw in order to fasten and loosen the screw. Screwdriver bits engage with the recesses and a torque is conveyed to the recesses from the screwdriver.
The recesses are arranged in minus shape, cross shape or Y shape at the top of the screw and are called minus screw, cross screw or Y screw.
Among these screws, a minus screw has a simple structure and shape, and has a good workability for the production. However, from the viewpoint of the strength of the screw, deep recesses, which extend to the total thickness of the screw head, cannot be formed. Shallow recesses each give a small engaging portion and small area for the screwdriver bit. Then, when the screwdriver bits are inserted into the recesses and fasten the screw, a large torque and force can not be attained. The screwdriver bits are easily disengaged from the recesses disposed in the screw head. As a result, the screwdriver bits spin free in the recesses of the screw head and damage the recesses, and the screwdriver can not fasten enough the screw. It happens unable to loosen the screw. Furthermore, it is difficult to insert the screwdriver into the recesses and causes an inefficient workability.
Recesses in a cross screw head each have a tapered cross section and the depth of the recesses can be made deeper so that the recesses each can ensure a wide engaging area with a screwdriver, and a large torque and force from the screwdriver is attained. Then, the fast and assured fastening and loosening become possible. The screwdriver can be easily inserted into the recesses and attain a good operation. However, when the screwdriver is inserted into the recesses of the cross screw and a large torque is applied to the screw, the screwdriver bits happen to rise up and disengage from the recesses, spin free, and damage the recesses.
The volume of the cross screw becomes small as the number and space of the recesses become larger. The engaging area of the recesses with the screwdriver bits becomes wide so that only small shear force is attained. Then, the structure and strength of the screw become weak and lower, respectively.
Recently, the screws utilized especially for home electric appliances, electric device parts, office machines such as copying machines, and optical apparatus or precision apparatus such as digital camera, tend to have a large head, and a thin and small size in order to attain a large fastening torque. The advantage and disadvantage of the cross or minus screw become more distinct.
In order to increase the torque to fasten the cross or minus screws, a vertical wall is disposed in one side of the recess and a spherical or inclined wall is disposed in the other side of the recess of the large screw head, respectively. In order to prevent users from disassembling the screws, there is a screw such that no torque is applied to the direction to loosen. Refer to JP,57-120714,A(1982).